This invention relates to supplying the signals to a large group of conductors in close proximity to another group of conductors in such a way that coupling noise between the groups is reduced. For example, this invention can be employed in supplying signals to the word lines of a Programmable Interconnect Array (PIA) in a high-density Programmable Logic Device (PLD) so as to reduce the noise coupled to the PIA bit lines by the signals on the word lines.
Wong et al. U.S. Pat. No. 4,871,930 is background to this invention and is hereby incorporated by reference herein.
A high-density PLD may include a PIA which generally comprises one or more Electrically Programmable Read-Only Memory (EPROM) arrays in which a large number of word lines drive bit lines via selectively programmed floating-gate transistors. Typically, all of the word lines cross the bit lines so that appropriate interconnections can be made. This causes parasitic capacitance to be present between the word lines and the bit lines. The PIA bit lines are therefore susceptible to coupling noise generated when large numbers of PIA word lines are switched coherently (i.e., simultaneously in the same direction). For example, when a global clear signal activates, or when large binary counters roll over, many macrocell outputs change simultaneously. As this happens, the states of a large number of word lines change simultaneously from logical one to logical zero or vice versa, thus generating a large transient electromagnetic field which is coupled to the bit lines. Furthermore, in high-density PLDs with dual feedback, the coupling noise is increased because when a set of N output macrocells switch coherently, 2N PIA word lines switch coherently. This is a consequence of the architecture of a conventional high-density PLD with dual feedback in which the output macrocell and its associated input/output (I/O) pad input both feed word lines in the PIA.
In view of the foregoing, it is an object of this invention to provide an improved technique for supplying signals to structures such as the PIA in high-density PLDs which reduces the noise coupled to the bit lines when large numbers of word lines switch coherently.
It is a further object of this invention to provide an improved technique for supplying signals to structures such as the PIA in high-density PLDs with dual feedback, which technique reduces the noise coupled to the bit lines when large numbers of word lines switch coherently.